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Measuring in Arc-Seconds
   Some USGS DEM data is stored in a format that utilizes three, five, or 30 arc-seconds of longitude and latitude to register cell values. The geographic reference system treats the globe as if it were a sphere divided into 360 equal parts called degrees. Each degree is subdivided into 60 minutes. Each minute is composed of 60 seconds. Said differently, an arc-second represents the distance of latitude or longitude traversed on the earth's surface while traveling one second (1/3600th of a degree).
   At the equator, an arc-second of longitude approximately equals an arc-second of latitude, which is 1/60th of a nautical mile (or 101.27 feet or 30.87 meters). Arc-seconds of latitude remain nearly constant, while arc-seconds of longitude decrease in a trigonometric cosine-based fashion as one moves toward the earth's poles. At 49 degrees north latitude, along the northern boundary of the Concrete sheet, an arc-second of longitude equals 30.87 meters * 0.6561 (cos 49) or 20.250 meters. Ideally, a three arc-second grid cell on the north edge of the Concrete sheet measures 60.75 meters along its north side and 92.60 meters along its east and west sides.


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